Several commercial anthelmintic drugs (e.g. levamisole, pyrantel, morantel, monepantel, derquantel) disrupt the acetylcholine signaling pathway by agonizing or antagonizing AChRs in parasitic gastrointestinal (GI) nematodes of ruminants. However, GI parasite populations that are resistant to traditional anthelmintics, including cholinergic anthelmintics, are spreading globally and cause significant losses in farm productivity and profitability each year. One strategy to overcome current and minimize or prevent new resistance is to develop an anthelmintic with a novel mode of action. Ongoing research continues to identify and characterize additional components of the ACh signaling pathway in nematodes. Based on the precedent for the anthelmintic effectiveness of drugs that target ACh signaling, other components of the ACh signaling pathway may be exploited as novel anthelmintic targets.
ACR-16 is a homopentameric, levamisole-insensitive nicotinic acetylcholine receptor (nAChR) similar to the human alpha-7 nicotinic AChR that is found in neuromuscular junctions of both free-living and parasitic nematodes. ACR-16 functions as a ligand-gated ion channel that likely regulates fast action of acetylcholine at neuromuscular junctions and in the nervous system. In 2005 and 2007, Touroutine et al. showed that a “gain-of-function” mutation in ACR-16 was lethal to the free-living nematode Caenorhabditis elegans (Cel), and that ACR-16-containing receptors account for all non-levamisole sensitive nicotinic synaptic signaling at the Cel neuromuscular junction. These observations suggest that ACR-16 plays a vital role in nematode physiology, which led us and several other research groups to hypothesize that an agonist or positive allosteric modulator of ACR-16 could impair parasitic nematode physiology and, thus, be an effective anthelmintic.
RIC3 (resistance to inhibitors of cholinesterase 3) is a chaperone protein that in humans is encoded by the RIC3 gene. The RIC3 gene was first discovered in C. elegans, and influences the maturation of various ligand-gated ion channels including the serotonin 5-HT3 receptor and nicotinic acetylcholine receptors, particularly the homomeric α7 nicotinic receptor. RIC3 enhances currents generated by these receptors by expediting receptor transport to the cell surface and by increasing receptor number. Before the instant disclosure, the importance of RIC3 expression in yielding functional ACR-16-containing channels in cells was not appreciated.
Haemonchus contortus (Hco), also known as the barber's pole worm, is a blood-feeding, parasitic GI nematode and one of the most pathogenic nematodes of ruminants. Adult worms attach to abomasal mucosa and feed on blood, which causes anemia, edema, and death of infected cattle, sheep and goats, mainly during summer months in warm, humid climates. Adult female Hco may lay over 10,000 eggs a day, which pass from the host animal in the feces. After hatching from their eggs, Hco larvae molt several times to the infective larval stage 3 (L3), which is ingested by host animals during grazing. Ingested larvae mature to the adult stage in the host GI tract and ultimately attach to the abomasal mucosa.
Hco infection, or haemonchosis, causes large economic losses for farmers globally, but, especially for those living in warmer climates. Anthelmintics are used to treat and control ruminant GI infections of Hco and other parasitic nematodes, but growing resistance of parasites to anthelmintics such as levamisole and monepantel has rendered nematode control strategies extremely challenging, if not ineffective. Accordingly, a critical need exists to develop new anthelmintics against GI parasitic nematodes with novel modes of action.
In addition to GI parasites of ruminants, novel anthelmintics are needed for other parasitic nematode species have developed resistance to commercial anthelmintics. One important example is Dirofilaria immitis (Dim), a filarial nematode transmitted by mosquitoes that causes heartworm disease in dogs, cats, ferrets, and wild canids. Heartworm disease is a serious and potentially fatal condition caused by the adult stage of Dim that affects the right side of the heart and pulmonary arteries. Heartworm larvae are transmitted year-round. Although heartworm disease is most prevalent in warmer climates (with higher mosquito populations), it has been diagnosed in all fifty US states and throughout southern Europe. Commercial heartworm disease preventives work by interrupting the parasite life cycle within the host, thereby preventing the development of adult worms and associated pathology. Only one chemical class, the macrocyclic lactones (MLs), is approved by the FDA and other global agencies for the prevention of heartworm disease.
Since 2005, the existence of Dim subpopulations that are resistant to MLs populations in the US has been confirmed. These populations appear to be localized, mostly to the Mississippi Delta region of the US, though transportation of pets and shelter dogs across state lines or even internationally puts other regions at risk. Both the inherent mechanism(s) of Dim resistance to MLs and the potential of ML resistance to spread are unknown. Thus, an urgent need exists to develop new heartworm disease preventives with novel modes of action.